Auxiliary power units (APUs) are utilized in a variety of applications in order to, for example, provide electrical power in instances in which the primary power supply is unavailable. For example, gas turbine APUs are installed onboard commercial transport airplanes to provide power to airplane systems during ground conditions in instances in which the main engines are not operating and in which external ground power supplies are not available.
Conventional gas turbine APUs are typically less fuel efficient than diesel engine powered external power carts or airline terminal supplied external power sources. In order to reduce the amount of fuel consumed on the ground, some airlines have tried to curtail use of onboard APUs. However, fuel burned by the APUs still remains a significant airline expense.
Hybrid electric power architecture has been applied to reciprocating piston engine powered road vehicles for the purpose of reducing fuel consumption. A series hybrid electric vehicle propulsion system typically consists of a reciprocating piston engine driving a generator, an electrical storage battery and an electric motor-generator coupled to the road wheels. By managing the distribution of electrical power flowing between the engine driven generator, traction motor and battery, the piston engine may be started and operated only when required. Intermittent operation of the engine allows the vehicle engine, during operation, to run at or near its most efficient speed and power loading, thereby reducing fuel consumption over the vehicle's operating cycle as compared to a conventionally powered vehicle with a continuously operating engine running at partial power load.
The piston engine on a series hybrid road vehicle shuts down while the vehicle operates on the battery, and then periodically restarts to replenish battery charge. However, such frequent starting and stopping of the gas turbine power section of an aircraft gas turbine APU is not likely to be practical due to constraints that are characteristic of gas turbine APU design. First, gas turbine APUs utilize a heavy main rotor that requires more time to spool up from a dead stop to operating speed as compared to a piston engine. Second, imposing repeated firing and cooling cycles on an APU turbine hot section may have a deleterious effect on the life the turbine hot section components. Reducing the frequency of starting and stopping cycles sufficiently to address turbine hot section thermal fatigue life issues may necessitate use of a battery or other energy storage device of such a large capacity that its weight is likely to be too great to be practical for aircraft use.
Simply loading and unloading a gas turbine APU while it runs at governed speed imposes lower thermal fatigue stress on the turbine section, but this would not reduce fuel consumption because specific fuel consumption of a continuously operating gas turbine engine increases substantially at partial load. Reducing APU rotor speed for the purpose of reducing total air flow through an idling APU has little benefit for reducing no-load fuel consumption because reducing speed also substantially reduces cycle pressure ratio, which reduces Brayton cycle efficiency, and consequently increases specific fuel consumption.